Abstract
Abstract. Land surface modellers need measurable proxies to constrain the quantity of carbon dioxide (CO2) assimilated by continental plants through photosynthesis, known as gross primary production (GPP). Carbonyl sulfide (COS), which is taken up by leaves through their stomates and then hydrolysed by photosynthetic enzymes, is a candidate GPP proxy. A former study with the ORCHIDEE land surface model used a fixed ratio of COS uptake to CO2 uptake normalised to respective ambient concentrations for each vegetation type (leaf relative uptake, LRU) to compute vegetation COS fluxes from GPP. The LRU approach is known to have limited accuracy since the LRU ratio changes with variables such as photosynthetically active radiation (PAR): while CO2 uptake slows under low light, COS uptake is not light limited. However, the LRU approach has been popular for COS–GPP proxy studies because of its ease of application and apparent low contribution to uncertainty for regional-scale applications. In this study we refined the COS–GPP relationship and implemented in ORCHIDEE a mechanistic model that describes COS uptake by continental vegetation. We compared the simulated COS fluxes against measured hourly COS fluxes at two sites and studied the model behaviour and links with environmental drivers. We performed simulations at a global scale, and we estimated the global COS uptake by vegetation to be −756 Gg S yr−1, in the middle range of former studies (−490 to −1335 Gg S yr−1). Based on monthly mean fluxes simulated by the mechanistic approach in ORCHIDEE, we derived new LRU values for the different vegetation types, ranging between 0.92 and 1.72, close to recently published averages for observed values of 1.21 for C4 and 1.68 for C3 plants. We transported the COS using the monthly vegetation COS fluxes derived from both the mechanistic and the LRU approaches, and we evaluated the simulated COS concentrations at NOAA sites. Although the mechanistic approach was more appropriate when comparing to high-temporal-resolution COS flux measurements, both approaches gave similar results when transporting with monthly COS fluxes and evaluating COS concentrations at stations. In our study, uncertainties between these two approaches are of secondary importance compared to the uncertainties in the COS global budget, which are currently a limiting factor to the potential of COS concentrations to constrain GPP simulated by land surface models on the global scale.
Highlights
Humanity has to face the urgency of climate change if it hopes to limit adverse future impacts (Allen et al, 2018; IPCC, 2019a, b)
We compared the simulated mechanistic Carbonyl sulfide (COS) fluxes at the global scale to former estimates; we studied leaf relative uptake (LRU) values estimated from monthly fluxes, which are pertinent for atmospheric studies, and compared them to monthly means of high-frequency LRU values
Uptake of modelled COS flux is around −8 pmol m−2 s−1 while field observations vary between −5 and 0 pmol m−2 s−1
Summary
Humanity has to face the urgency of climate change if it hopes to limit adverse future impacts (Allen et al, 2018; IPCC, 2019a, b). In order to make reliable predictions of future climate, scientists have built powerful numerical Earth system models (ESMs), where they continuously integrate gained knowledge on a multitude of climate-related and climate-interacting processes. In the global carbon budget, the land component shows the largest uncertainty (Le Quéré et al, 2018; Bloom et al, 2016). Land surface models (LSMs) struggle to accurately represent the large spatial and temporal variability of the CO2 gross and net fluxes (Anav et al, 2015). The quantity of assimilated carbon is called gross primary productivity (GPP). All other carbon fluxes and stocks derive from this first gross assimilation flux. To help reduce uncertainties in the estimated GPP, LSMs can benefit from knowledge obtained through local eddy covariance measurements of the net ecosystem–atmosphere CO2 exchange (Friend et al, 2007; Kuppel et al, 2014)
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